1. Field of the Invention
The present invention relates to a projection display apparatus having a polarized beam splitter for receiving a light from a light valve and analyzing a modulated light. More specifically, the present invention relates to an improvement on the polarized beam splitter.
2. Description of the Related Art
In Published Japanese Patent Registration No. 2599309, disclosed is a color projection display apparatus for separating a light from a light source into respective color lights of R, G and B by a color-separation optical system, making incident each of the color lights on a polarized beam splitter to be polarized and separated, making incident one of lights obtained by polarizing and separating a color light on a reflection light valve arranged for each color light to be modulated, making incident emitted reflected lights including the modulated lights on the polarized beam splitter, analyzing the same and thereby extracting the modulated lights, composing such analyzed modulated lights with one another by a composing optical system and then projecting the composed light by a projection optical system.
In FIG. 7, shown is a structure of the projection display apparatus disclosed in the Published Japanese Patent Registration No. 2599309.
Specifically, a light emitted from a light source 71 is made incident on a dichroic mirror 72 arranged as a "color-separation optical system" on an optical axis. Then, the light is subjected to a color-separation into a B light to be transmitted and R and G lights to be reflected according to a dichroic characteristic of the mirror 72. The transmitted B light is made incident on a polarized beam splitter 74B for the B light as a "polarization and separation optical system". An S polarized light of the B light reflected by a polarizing and separating section of the polarized beam splitter 74B is made incident on a reflection light valve 75B.
On the other hand, a mixed light of the reflected R and G lights is made incident on a dichroic mirror 73 arranged as a "color-separation optical system" on the optical axis in parallel with the dichroic mirror 72. Then, the mixed light is subjected to a color-separation into a G light to be reflected and an R light to be transmitted according to a dichroic characteristic of the mirror 73.
The G light obtained by the color-separation is made incident on a polarized beam splitter 74G as a "polarization and separation optical system". An S polarized light reflected by a polarizing and separating section of the polarized beam splitter 74G is made incident on a light valve 75G for the G light. Likewise, the R light is made incident on a polarized beam splitter 74R as a "polarization and separation optical system". Then, an S polarized light reflected by a polarizing and separating section of the polarized beam splitter 74R is made incident on a light valve 75R for the R light.
The S polarized lights respectively made incident on the light valves 75B, 75G and 75R are modulated by signals applied to the same, reflected and emitted as lights including modulated and unmodulated lights. These lights are then made incident on the polarized beam splitters 74B, 74G and 74R for the respective colors, and subjected to an analysis by the polarizing and separating sections of the polarized beam splitters 74B, 74G and 74R. Only the modulated lights are extracted as P polarized lights transmitted through the polarized beam splitters 74B, 74G and 74R, and the analyzed lights are color-composed by a dichroic mirror 76 and a dichroic mirror 77 arranged as a "composing optical system". Then, a result of the color composing is projected to a projection lens 78 as a "projection optical system".
In the projection display apparatus disclosed in the foregoing Published Patent Gazette, as described above, the dichroic mirrors 76 and 77 are used as the composing optical system. Another apparatus has also been disclosed, where a cross dichroic prism is used as "composing optical system".
The inventors of the present invention have investigated a significant problem inherent in the foregoing conventional projection display apparatus, which is constructed in a manner that the reflection light valves 75B, 75G and 75R are arranged for the respective colors, modulated lights among lights reflected by the light valves 75B, 75G and 75R are analyzed by the polarized beam splitters 74B, 74G and 74R arranged for the respective colors and then the analyzed lights are color-composed. Specifically, for an image projected on a screen by the projection optical system (projection lens 78), it was impossible to make registration adjustment (pixel positioning) coincident among the colors. Consequently, pixel deviation occurred.
Usually, with reference to the pixel positioning, relative to projected images from one of the light valves 75B, 75G and 75R for specified color lights, pixel deviation of specified positions of the other light valves 75B, 75G and 75R for the other colors must be limited to 1/2 pixel or lower, preferably within 1/3 pixel on full surfaces of the projected images.
A level of pixel deviation which is not a problem at all for the conventional light valves 75B, 75G and 75R, each of these having a pixel size of about 40 .mu.m, becomes a severe problem for a light valve having a very small pixel size of about 10 .mu.m.
Further, as a projected image is increased in size to be displayed on a large screen, the foregoing problem of pixel deviation will become more severe.
The inventors found as a result of extensive investigation that the problem of pixel deviation is not a problem that is created after execution of a vibration test or an environmental test such as a temperature cycle for the projection display apparatus. Rather, this is a basic problem which is created at the time of assembling the constituent members of the projection display apparatus. The inventors found a characteristic of the problem is that although an original shape of the display section of the light valve is rectangular, the display section is deformed to be a parallelogram, and consequently, pixel deviation occurs in a projected light from the light valve for a specified color light.
Furthermore, the inventors investigated projected images by replacing, among the constituent members of the projection display apparatus, the members for the respective light colors. When an experiment was made by replacing the polarized beam splitter with another for the other color light and arranging the same, a projected image of another color light was also projected in a parallelogram of the same size. Therefore, it was discovered that the projection of the image in the parallelogram rather than in the original rectangular shape can be attributed to the polarized beam splitter.